Photosynthesis is limited by the conductance of carbon dioxide (CO 2 ) from intercellular spaces to the sites of carboxylation. Although the concept of internal conductance (g i ) has been known for over 50 years, shortcomings in the theoretical description of this process may have resulted in a limited understanding of the underlying mechanisms. To tackle this issue, we developed a three-dimensional reaction-diffusion model of photosynthesis in a typical C 3 mesophyll cell that includes all major components of the CO 2 diffusion pathway and associated reactions. Using this novel systems model, we systematically and quantitatively examined the mechanisms underlying g i . Our results identify the resistances of the cell wall and chloroplast envelope as the most significant limitations to photosynthesis. In addition, the concentration of carbonic anhydrase in the stroma may also be limiting for the photosynthetic rate. Our analysis demonstrated that higher levels of photorespiration increase the apparent resistance to CO 2 diffusion, an effect that has thus far been ignored when determining g i . Finally, we show that outward bicarbonate leakage through the chloroplast envelope could contribute to the observed decrease in g i under elevated CO 2 . Our analysis suggests that physiological and anatomical features associated with g i have been evolutionarily fine-tuned to benefit CO 2 diffusion and photosynthesis. The model presented here provides a novel theoretical framework to further analyze the mechanisms underlying diffusion processes in the mesophyll.

The theoretical upper limit for the operational efficiency of plant photosynthesis has been estimated from a detailed stepwise analysis of the biophysical and biochemical subprocesses to be about 4.6% for C 3 and 6.0% C 4 plants (Zhu et al., 2008(Zhu et al., , 2010. (These estimates assume a leaf temperature of 30°C and an atmospheric [CO 2 ] of 387 ppm and were calculated relative to the full solar spectrum at the earth's surface. These efficiencies would be slightly more than double if calculated relative to only the photosynthetically active radiation [i.e. 400-700 nm; Zhu et al., 2008].) While estimates of the theoretical upper limit of photosynthetic efficiency in microalgae have not been conducted as systematically, the same considerations apply as for plants, although microalgae may have a lower respiratory rate that would raise the upper limit for the efficiency of net photosynthesis accordingly (Melis, 2009). Microalgae expressing bicarbonate transporters will mimic the higher efficiency of C 4 plants due to suppression of photorespiration, while those that do not actively concentrate CO 2 would have upper efficiency limits similar to C 3 plants. The highest short-term efficiencies observed for plants in the field, assessed from maximum growth rates, are about 3.5% for C 3 and 4.3% for C 4 plants, and these drop further to 2.4% and 3.4% when calculated over a full growing season (Monteith, 1977;Piedade et al., 1991;Beale and Long, 1995). For crop plants, these highest full growing season efficiencies are those that define the yield potential (Fischer and Edmeades, 2010) or record yield of the crop and are at least twice greater than photosynthetic efficiencies observed under most commercial farming conditions.The primary reason why the highest observed photosynthetic efficiencies are 30% or more lower than theoretical efficiencies is light saturation of photosynthesis. Photosynthesis responds nonlinearly to increases in insolation. For example, C 3 leaf photosynthesis is saturated by approximately 25% of maximum full sunlight, and light intercepted above this amount will lower photosynthetic efficiency in proportion to the excess light absorbed (Fig.

The contradiction between “high yielding” and “early maturing” hampers further improvement of annual rice yield. Here we report the positional cloning of a major maturity duration regulatory gene,Early flowering-completely dominant(Ef-cd), and demonstrate that natural variation inEf-cdcould be used to overcome the above contradictory. TheEf-cdlocus gives rise to a long noncoding RNA (lncRNA) antisense transcript overlapping theOsSOC1gene.Ef-cdlncRNA expression positively correlates with the expression ofOsSOC1and H3K36me3 deposition. Field test comparisons of early maturingEf-cdnear-isogenic lines with their wild types as well as of the derivative early maturing hybrids with their wild-type hybrids conducted under different latitudes determined that the early maturingEf-cdallele shortens maturity duration (ranging from 7 to 20 d) without a concomitant yield penalty.Ef-cdfacilitates nitrogen utilization and also improves the photosynthesis rate. Analysis of 1,439 elite hybrid rice varieties revealed that the 16 homozygotes and 299 heterozygotes possessingEf-cdmatured significantly earlier. Therefore,Ef-cdcould be a vital contributor of elite early maturing hybrid varieties in balancing grain yield with maturity duration.